In the photoreceptor layer of Rana pipiens, cGMP levels have been measured in vivo using a rapid freezing technique. In darkness mean concentrations of 57 pmol kg-1 were obtained, but after extended periods of illumination levels dropped to 9 pmol kg-1. Brief (100 msec) flashes produced barely significant (10- 15%) declines; minimum concentrations were reached only after several hours of dim illumination. The results suggest that the bulk of cGMP decline reflects a long term biochemical readjustment of rods to background illumination rather than a phototransductive message. In the outer plexiform layer of the cat retina background stimuli enhance the responses of horizontal cells to small red flickering spots. The responses to the spots originate with the red cones; the enhancement effect of the background originates with rods, as demonstrated by spectral sensitivity studies. The effect diminishes approximately exponentially with the width of either square or slit stimuli (space constant 250-400 um), but is larger with squares, both results consistent with theoretical models of voltage attenuation of background induced surround signals which decrement into the test region through the horizontal cell syncytium. The enhancement appears to result from the direct action of the horizontal cells on cones. There are also suggestions of similar horizontal cells on cones. There are also suggestions of similar horizontal cell mediated interactions among different cone types. The effects of light on the input resistances of on-beta ganglion cells have been measured in a perfused cat eyecup. Dark resistances were typically about 45 megohms, and all components of the light evoked response produced a small decrease in resistance. Reversal potentials for depolarizing response components lay positive to resting levels, but between values typical of ionic Nernst potentials, suggesting that both excitatory and inhibitory ionic drives were simultaneously active. Off hyperpolarization reversed negative to the restin potential. The results relate to electron microscopic studies of synaptic impingement onto on-beta cells.